Microwave energy is being used increasingly in a variety of manufacturing operations, especially those involving the curing of polymeric materials. Microwave processing of polymeric materials is advantageous for a number of reasons. The application of microwave irradiation decreases the time required to cure some polymers as compared with conventional heating methods. This is because the volumetric deposition of microwave irradiation is more efficient than conduction from the surface resulting from conventional heating techniques. See, for example, Polymer Curing In A Variable Frequency Microwave Oven, R. J. Lauf et al., Oak Ridge National Laboratory. See also, U.S. Pat. No. 5,296,271 to Swirbel et al., which proposes a method of curing photoreactive polymers by exposing them to microwave irradiation. Additionally, microwave processing is more economically attractive than conventional heating techniques due to the shorter processing time required to cure the resin.
The quality of a resin product is dependent on, among other things, adequate resin flow, gelation, pressure, temperature and cure rate. Unfortunately, it is difficult to determine whether these parameters have been adequately achieved during many processes such as molding or curing. This is because the product typically is hidden from view during processing, and because the manifestation of inadequate processing, can only be detected if the interior of the resin product can be viewed. It is desirable, therefore, to be able to perform diagnostic tests on resin products during manufacturing to determine various information about the physical and chemical state that is otherwise obscured from view.
It is known to use microwave energy for measuring various characteristics or properties of materials, including moisture content, thickness, magnetic anisotropy, and dielectric anisotropy. For example, U.S. Pat. No. 3,458,808 to Agdur discloses an apparatus for measuring various properties of a material via microwave resonance techniques and measuring the time interval between the resonant frequency peaks of a single mode microwave cavity. U.S. Pat. No. 4,904,928 to Lewis, discloses measuring variations in material properties by observing changes in the resonant frequency of various modes, relative to one another, within a single mode microwave cavity.
Unfortunately, resonance techniques are able to monitor only characteristics or properties within a localized area of a product. Furthermore, techniques based on the resonance condition of a microwave cavity cannot generate information about an entire product. However, it is desirable to obtain volumetric information about various properties of a workpiece during the manufacturing process. It is also desirable to provide real-time feedback of these properties to enable greater control over manufacturing and fewer rejected products during manufacturing.